The present invention is primarily directed to coal fired furnaces although it will be understood by the artisan that a similar system can also be used on oil fired furnaces, hybrid coal/oil furnaces, gas fired furnaces and furnaces which burn other materials. It should therefore be understood that while the description which follows is directed to pulverized coal burning furnaces, that is by way of example only and not a limitation. The present invention is generally applicable in any furnace requiring excess air to assure complete combustion or furnaces requiring excess air as load is reduced.
Typical prior art burner arrangements for a furnace comprises a burner or coal nozzle through which pulverized coal and primary air are introduced into the furnace. The primary air typically supplies only about 20 percent of the air needed to fully oxidize the fuel. Therefore, each burner nozzle is provided with a secondary air supply. The secondary air supply typically consists of a "windbox" or air plenum that is in communication with a burner register. The prior art burner registers are of two general types. The first type utilizes pivoting slat vanes journaled between two ring members to form a band The coal nozzle is centrally located along the axis of the band. The vanes pivot from a fully closed position, where the end of one slat coincides with the beginning of the next thus forming a closed ring about the fuel nozzle, to an open position where the vanes are positioned generally radially with respect to the fuel nozzle, thus permitting the free flow of secondary air. Such a register utilizes a single assembly to perform the dual functions of controlling both the volume and direction of the secondary air supply. An example of such a register is taught in Chapter 9 of "Steam/Its Generation and Use" by the Babcock and Wilcox Company, 1978 Edition.
The second type of prior art secondary air register is formed from a plurality of movable, radial "pie" shaped wedges which in the closed position form a closed circular valve surface and which, as opened, operates to control the volume and direction of secondary air introduced from an associated windbox into the furnace along with the fuel and primary air. These prior art arrangements suffer from various deficiencies (discussed below) which the present invention has overcome.
Owing to their considerable number of required interrelated moving parts, which were subjected to the severe environmental conditions existing in the space adjacent to the furnace, the prior art registers were unreliable and subject to frequent and costly repair efforts. These repairs would require the shutting down of the furnace facility at considerable expense and inconvenience to the operator of the furnace (usually an electrical generation utility).
In addition, the prior art registers introduced secondary air into the furnace in a turbulent but generally random pattern with only a small and ineffective swirl component. This led to the incomplete combustion of the fuel and to erosion of the furnace walls in the vicinity of the burner due to the action of deposits of only partially burned fuel along the furnace walls. Moreover, the prior art registers required the introduction of large amounts of secondary air even when the burner was idle in order to protect the register from heat damage. This required that the furnace be equipped with the capacity to generate and otherwise process large amounts of secondary air and led to the problem of erosion or wear damage occurring in the various furnace components exposed to the higher velocity air flow (e.g., fans, registers, heat exchangers, superheaters, etc. It is noted that the wear resulting from the gas flow against the elements in the flow path is a function of the cube of the gas velocity.
The operation of prior art registers resulted in inefficient furnace operation, especially at low loads. Moreover, due to the lack of a well defined fuel/air flow pattern, there was a tendency in prior art furnaces for the intense heat and pressure variations existing in the furnace to cause the fuel from the nozzle to be "blown" against the relatively cold furnace walls. This resulted in poor combustion of the coal and additional damage to the furnace walls.